A large current is required to drive a traction motor used in a hybrid car and so forth. For example, when starting or accelerating, a hybrid car requires a huge current equal to or greater than 100 A to drive the traction motor. As this power supply apparatus for supplying a large current, a power supply apparatus including secondary cells with a high energy density, has been known.
In addition, as this power supply apparatus, a power supply apparatus in which a plurality of unit cells, such as nickel-hydrogen batteries, nickel-cadmium batteries or lithium-ion batteries are connected, has been known (for example, see Patent Literatures 1 and 2).
The power supply apparatuses disclosed in Patent Literatures 1 and 2 have a plurality of connected unit cells and a holder to secure these unit cells. The material for the holder is, for example, plastic. Even if one unit cell fails, a power supply apparatus having a plurality of unit cells as described above can supply a current as long as other unit cells work.
However, in the power supply apparatuses as disclosed in Patent Literatures 1 and 2, a material having a low thermal conductivity, such as plastic, is used to fabricate a holder, the heat of unit cells generated during the operation of the power supply apparatus is less likely to be conducted to the holder. Therefore, there has been a problem that heat is accumulated in unit cells during the operation of the power supply apparatus, and therefore the temperature of the unit cells increases.
When the temperature of unit cells increases, the performance of the unit cells decreases, and therefore the performance of the overall power supply apparatus decreases. Meanwhile, if a unit cell is a lithium-ion battery, increase in temperature involves a risk of thermal runaway.
To solve this problem, a technique for enclosing a plurality of connected unit cells with a molded article made of metal such as aluminum having a high thermal conductivity (for example, see Patent Literature 3) has been known.
FIG. 1 is a perspective view showing power supply apparatus 1 disclosed in Patent Literature 3. As shown in FIG. 1, power supply apparatus 1 has molded article 10 having a plurality of unit cell accommodating cavities 11 and unit cells 20 accommodated in unit cell accommodating cavities 11. In addition, molded article 10 has coolant channel 13 that allows coolant for cooling molded article 10 to pass through. Molded article 10 is made of a material such as aluminum having a high thermal conductivity.
In this way, by enclosing unit cells 20 with molded article 10 having a high thermal conductivity, the heat of unit cells 20 generated during the operation of power supply apparatus 1 is conducted to molded article 10 and removed. Therefore, unit cells 20 are cooled to prevent the temperature of unit cells 20 from increasing. In addition, heat conducted to molded article 10 is conducted to coolant flowing through coolant channel 13 and discharged outside.
However, power supply apparatus 1 shown in FIG. 1 has a problem that unit cells 20 are not sufficiently cooled during the operation, and therefore the temperature of unit cells 20 increases. Now, referring to FIG. 2, a cause for increase in temperature of unit cells 20 during the operation of power supply apparatus 1 will be explained. FIG. 2 is an enlarged view showing region X in power supply apparatus 1 shown in FIG. 1.
In power supply apparatus 1, unit cells 20 are accommodated in unit cell accommodating cavities 11, so that the diameter of unit cell accommodating cavity 11 is set greater than the diameter of unit cell 20. Therefore, when unit cell 20 is accommodated in unit cell accommodating cavity 11, gap G occurs between unit cell 20 and the inner wall of unit cell accommodating cavity 11 as shown in FIG. 2. When gap G is formed between unit cell 20 and the inner wall of unit cell accommodating cavity 11, the air between unit cell 20 and unit cell accommodating cavity 11 serves as a heat insulator, so that the heat of unit cell 20 is not conducted to the body. Therefore, heat remains in unit cell 20 to increase the temperature of unit cell 20.
Therefore, even if the heat conductivity of the material for the molded article enclosing unit cells is made high, it is not possible to sufficiently cool the unit cells, and this causes problems of decrease in the performance of unit cells and thermal runaway.
To solve the problem like this, a power supply apparatus has been known where not unit cells, but electrode groups each composed of a cathode, an anode and a separator, and electrolytic solution are directly accommodated in a molded article (see Patent Literature 4). FIG. 3 is an exploded perspective view showing a power supply apparatus disclosed in Patent Literature 4. As shown in FIG. 3, power supply apparatus 1 has electrically conductive molded article 10 having four accommodating cavities 11 and made of aluminum and so forth, and four electrode groups 21 accommodated in accommodating cavities 11. In addition, the inside of molded article 10 is hollow to allow cooling air to pass through. Moreover, in power supply apparatus 1 shown in FIG. 3, electrode groups 21 are serially connected through side plates 31 and 33.